Various types of optical displays are commonly used in a wide variety of applications. Included among these various types of optical displays are liquid crystal displays (LCDs) such as Active Matrix LCDs (AMLCDs). LCDs typically use a passive or active matrix display grid to form an image on the display surface. The luminance of the LCD is provided by backlight lamp. One of the attractive characteristics of many LCD displays is the flexibility of configuring a system with the particular features needed for a specific application.
One important performance parameter in certain LCD displays is the range of luminance that can be provided by the LCD display, commonly referred to as the dimming range. In many applications it is critical that a display make information clearly visible in a wide variety of ambient light conditions. For example, a display used in an avionics control system will need to display information to the pilot under lighting conditions that can range from near total blackness to the extreme glare created by facing directly into daytime sunlight. Such a display must have a high maximum dimming ratio, where the dimming ratio is the ratio of the display luminance at highest brightness to the display luminance at its lowest setting. Without a sufficiently high dimming ratio, a viewer of the display may be unable to easily read information from the display in high ambient light conditions, low ambient light conditions, or both. In some applications, the required maximum dimming ratio may be as little as 100:1. In other applications, a maximum dimming ratio of 20,000:1 or greater may be required to effectively display information in its expected range of ambient conditions. Thus, in many applications the display must be able to accurately and clearly display information through a wide dimming range, with the ability to precisely control the amount of dimming.
LCD and other types of optical displays use a fluorescent lamp as a light source to illuminate the display. The lamp is driven by a lamp driver circuit that powers the lamp and controls the lamp output. One way to provide a wide dimming range is for the lamp driver circuit to drive the lamp at different luminance levels. Unfortunately, it can be difficult for a lamp driver circuit to accurately drive the fluorescent lamp over the entire range of needed different luminance levels without experiencing performance difficulties such as low efficiency and uneven luminous output.
For example, in a lamp driver that drives a fluorescent lamp flicker at low luminance levels can be a problem. Specifically, during low luminance operation of the fluorescent lamp conditions in the plasma can exist such that for a section of plasma longitudinal e-fields and the mean free path of electrons in a section of discharge correspond to electrons with the exact kinetic energy required to transition mercury ions to a higher energy state. Electrons in the plasma section thus go from elastic collisions to inelastic collisions with the mercury ions causing the plasma column to appear as sudden negative impedance to the fluorescent lamp driver. This drastic perturbation causes the fluorescent lamp driver to adjust itself to the radical load change, and the plasma section immediately reverts back to elastic collisions between electrons and mercury atoms because of changes in the longitudinal e-fields due to driver adjustment, leading to oscillations between the two states. These oscillations will be perceived as subtle flickering in the luminous flux coming from the lamp and may be unacceptable in high end avionics applications.
Thus, what is needed is an improved lamp driver system that provides a wide luminance range and precise brightness control while providing good efficiency.